1. Field of the Invention
The present invention relates to a production method for a polycrystalline thin film having the crystal structure of a C-type rare earth oxide with a well-ordered crystal alignment and a production method for an oxide superconducting element with superior superconductive properties that comprises the polycrystalline thin film and an oxide superconductive layer provided thereon.
Priority is claimed under 35 U.S.C. 119 to Japanese patent application No. 2002-226625, filed on Aug. 2, 2002, which is incorporated herein by reference.
2. Description of the Related Art
Oxide superconducting elements discovered in recent years are superior superconductors that exhibit a critical temperature that exceeds the temperature of liquid nitrogen, but presently the various problems described below must be solved in order to use this type of oxide superconducting element as a practical superconductor. One of the problems is that the critical current density of the oxide superconducting element is low. The problem of the low critical current density of the oxide superconducting element is largely caused by the anisotropic electrical properties intrinsic to the crystal of the oxide superconducting element. In particular, it is known that although electricity flows readily along the a-axis direction and the b-axis direction of the oxide superconducting element, it does not flow readily along the c-axis direction. Thus, in order to form an oxide superconducting element on a substrate and use it as a superconductor, it is necessary to form an oxide superconducting element that has a good crystal orientation on the substrate. Furthermore, it is necessary to align the a-axis and the b-axis of the crystal of the oxide superconducting element in the direction in which the electricity has to flow and to align the c-axis of the oxide superconducting element in another direction.
Thus, conventionally, a sputtering apparatus is used when forming the oxide superconducting element on a substrate such as a metal tape, and an intermediate layer that has a good crystal orientation consisting of MgO, SrTiO3, or the like, is formed on the substrate beforehand. However, when the oxide superconducting layer is formed on a monocrystal substrate comprising these materials, the critical current density is several ten-thousands of A/cm2, while in contrast, the oxide superconducting layer formed on the intermediate layer described here only attains a critical current density having the extremely low value of about 1,000 to 10,000 A/cm2. This is thought to occur for the following reasons.
FIG. 11 shows the cross-sectional structure of an oxide superconducting element wherein an intermediate layer 2 is formed on a substrate 1 of a polycrystalline material such as a metal tape, and an oxide superconducting layer 3 is formed as a layer by a sputtering method on this intermediate layer 2. In the structure shown in FIG. 11, the oxide superconducting layer 3 is in a polycrystalline state, where numerous crystal grains 4 are randomly bonded. Individually, the c-axis of each of the crystal grains 4 exhibits a perpendicular orientation with respect to the surface of the substrate 1, while the a-axis and the b-axis are oriented in random directions.
It is thought that when the a-axis and the b-axis of each of the crystal grains that form the oxide superconducting layer are randomly oriented, the quantum coupling in the superconducting state is lost at the crystal grain boundaries where the crystal orientations have become disrupted, and as a result, this causes a deterioration in the superconductive properties, in particular, the critical current density. In addition, it appears the cause of the oxide superconducting element developing into a polycrystalline state in which the a-axis and the b-axis are not aligned is that the intermediate layer 2 formed therebelow is in a polycrystalline state randomly oriented along the a-axis and the b-axis. In other words, it appears that the oxide superconducting layer 3 grows in conformity to the crystals of the intermediate layer 2 as a result of the oxide superconducting layer being formed on an intermediate layer 2 that has such a state.
Thus, the inventor discovered that if first an intermediate layer of yttria-stabilized zirconia (ZrO2—Y2O3; abbreviated YSZ), which has a good orientation along the a-axis and the b-axis, is formed in advance on a polycrystalline substrate using a special technique, and an oxide superconducting layer is then formed on this intermediate layer, it becomes possible to produce an oxide superconducting element that exhibits a good critical current density. The inventor has filed patent applications in connection with this technology, such as Japanese Patent Application, First Publication No. Hei 06-145977, Japanese Patent Application, First Publication No. Hei 09-120719, and Japanese Patent Application, First Publication No. Hei 10-121239.
In the technology disclosed in these patent applications, when the YSZ film consisting of the desired constituents is formed on a polycrystalline substrate by sputtering a base material (referred to as the target) consisting of YSZ, crystals of YSZ having an unfavorable crystal orientation can be selectively eliminated by concurrently carrying out an ion beam assist, in which an ion beam such as Ar+ is irradiated from an oblique direction with respect to the film forming surface of this polycrystalline substrate. The crystals of YSZ having a good crystal orientation can be thereby selectively deposited. Thus, it is possible to form an intermediate layer of YSZ having a superior orientation.
As described above, according to the technology disclosed in applications previously filed by the inventor, it is possible to obtain a polycrystalline thin film of YSZ having favorably oriented a-axes and the b-axes, and it is possible to verify that the oxide superconducting element formed on this polycrystalline thin film exhibits a strong critical current density. Thus, the present inventor initiated investigations of technology for producing an intermediate layer comprising a more preferable polycrystalline thin film from different materials.
FIG. 12 shows the cross-sectional structure of an example of an oxide superconducting element that the inventor has recently used. The oxide superconducting element D has a four-layer structure made by forming an intermediate layer 6 for orientation control comprising YSZ or MgO on a metal tape base 5 using the technology explained above; next a reaction inhibiting intermediate layer 7 comprising Y2O3 is formed; and then the oxide superconducting layer 8 is formed thereon.
The reason for using this type of four-layer structure is to prevent diffusion reactions that occur between the intermediate layer consisting of YSZ and the oxide superconducting layer having a YBa2Cu3O7−x (0<X<0.5) composition. Specifically, in order to obtain the oxide superconducting layer having a YBa2Cu3O7−x (0<X<0.5) composition, after forming the oxide superconducting layer having the objective composition by using a film forming technique such as sputtering, temperature treatment must be carried out by heating the oxide superconducting layer to several 100 degrees. Due to the heat supplied during this heat treatment, a diffusion reaction of elements is promoted between the intermediate layer consisting of YSZ and the oxide superconducting layer having a YBa2Cu3O7−x (0<X<0.5) composition. Thus there is a concern that the superconducting properties will deteriorate. However, this four-layer structure effectively contributes to preventing this.
The YSZ crystal that forms the intermediate layer 6 for orientation control has a cubic system crystal structure, while the oxide superconducting layer having a YBa2Cu3O7−x (0<X<0.5) composition has what is termed a perovskite structure. While both are a type of face-centered cubic structure and have similar crystal lattices, there is a difference of about 5° in the grid interval of their crystal lattices. For example, in the case of YSZ, the distance between the nearest atoms, specifically, the atom positioned at a corner of the cubic lattice and the atom positioned at the center of a face of the cubic lattice, is 3.65 Å (where 10 Å=1 nm). This same distance between nearest atoms for Yb2O3 is 3.69 Å, and this same distance between nearest atoms for the oxide superconducting layer having a YBa2Cu3O7−x (0<X<0.5) composition is 3.81 Å. The distance between nearest atoms for Yb2O3 shows an intermediate value between those of the YSZ and YBa2Cu3O7−x (0<X<0.5). Thus Yb2O3 is thought to be effective for bridging the difference in the sizes of the lattices and to be effective as an intermediate layer for preventing reactions due to their compositions being similar.
However, as shown in FIG. 12, in the four-layer structure, the number of necessary layers becomes large, and thus there the problem that the number production steps increases.
Thus, with the object of forming a reaction-preventing intermediate layer 7 having a good orientation directly on a polycrystalline base 5, the inventor attempted to form a polycrystalline thin film to serve as the reaction preventing intermediate layer 7 on the polycrystalline base 5 using ion beam assist technology disclosed in patent applications previously filed by the inventor. This polycrystalline thin film comprises crystal grains of an oxide having the crystal structure of a C-type rare earth oxide represented by any of the formulas: Y2O3, Sc2O3, Nd2O3, Sm2O3, Eu2O3, Gd2O3, Tb2O3, Dy2O3, Ho2O3, Er2O3, Yb2O3, Lu2O3, and Pm2O3, and has grain boundary inclination angles, formed between the same crystal axes of the crystal grains along the plane parallel to the film forming surface of the polycrystalline base 5, that are controlled to within 30°.
As a result, the inventor discovered that by first using the ion beam assist technology disclosed in the patent applications, it is possible to form the polycrystalline thin film consisting of the crystal structure of a C-type rare earth oxide, such as Y2O3 or the like, having a superior crystal orientation wherein the grain boundary inclination angle is controlled to within 30° or less. This technology is disclosed in the filed Japanese Patent Application, First Publication No. 2001-114594.
While this patent application reported about a polycrystalline thin film consisting of mainly Y2O3, the possibility of polycrystalline thin films having different C-type rare earth oxide crystal structures was only pointed out. Thus, with respect to the polycrystalline thin films having each of the types of the C-type rare earth oxide crystal structures described above, further research would be required to discover what production conditions have a significant influence on the crystal orientation properties thereof.
The technology for forming various types of well-oriented films on a polycrystalline substrate has also been widely used outside the field of application of the oxide superconducting elements described above. For example, it is used in the fields of optical thin films, magneto-optical disks, circuit wiring boards, high-frequency waveguides, high-frequency filters, and cavity resonators. In all of these technologies as well, forming well-oriented polycrystalline thin films having a stable film quality on a substrate using these technologies remains a difficult task. That is, even more preferably, improved crystal orientation properties of a polycrystalline thin film would be advantageous if utilized with the optical thin film, magnetic thin film, circuit film or the like formed directly thereon.
The present invention has an object of providing a production method for a polycrystalline thin film. The present invention was completed as a result of numerous studies in which a polycrystalline thin film having the crystal structure of a C-type rare earth oxide with a good crystal orientation was formed on a polycrystalline substrate by applying the ion beam assist previously provided by the inventor. Among the polycrystalline thin films having the crystal structure of the C-type rare earth oxide described above, specifically, the polycrystalline thin films having the crystal structure of C-type rare earth oxides represented by the formulas for the six types: Sm2O3, Gd2O3, Y2O3, Ho2O3, Er2O3, and Yb2O3, materials having a good crystal orientation were chosen from among these six by forming films while varying the assisting ion beam energy applied when the polycrystalline thin film was formed and varying the temperature of the polycrystalline substrate on which the polycrystalline thin film was deposited. Among the production methods for polycrystalline thin films, changes in ion beam energy and temperature are thought to influence the crystal orientation properties significantly. Thereby, the favorable production conditions for these selected materials were discovered.
In addition, another object is to provide a production method for an oxide superconducting element that provides a polycrystalline thin film formed after determining the advantageous ion beam energy and the temperature for the polycrystalline substrate, and providing an oxide superconducting layer that provides an oxide superconducting element layer having a superior crystal orientation on this polycrystalline thin film.